Irony is defined as being a situation in which the reader recognizes a deeper meaning to something that is said or that happens within a story than do the characters of the story.
On page 44 of my edition of A Separate Peace, Gene is questioning Finny about what his reaction would be if Gene took the prize for best academic student in the class. After an extended exchange, Finny ends the discussion with the comment, "I'd kill myself out of jealous envy." Gene accepts this statement as being true and is thunderstruck by Finny's deceptive nature; an ironic situation because Finny had no interest in trying to be a good student, didn't care about Gene's academic success at that time, and later actively pushed Gene to be a better student.
On page 89, some of the Devon boys have been shoveling the train tracks in order to allow the trains to resume.
All of us lined both sides of the track and got ready to cheer the engineer and passengers. The coach windows were open and the passengers surprisingly were hanging out; they were all men, I could discern, all young, all alike. It was a troop train.
The schoolboys were working to enable boys only slightly older than themselves to be taken to the war, which they were facing themselves at the conclusion of the school year. While the schoolboys were tired and dirty from their hard work shoveling the grimy snow, at that time the troops were rested and fresh in crisp, clean uniforms. They had a purpose to their lives, while the students at Devon seemed to be living in limbo, with very little real reason for their daily activities beyond shoveling snow to allow troop trains to get through.
I'm not sure on what page, but in Chapter 8 when Gene prtests to Finny that there will not be an Olympics in 1944 due to the war, Finny responds, "Leave your fantasy life out of this. We're grooming you for the Olympics, pal, in 1944."
The irony occrus when Finny responds, "Leave your fantasy life out of this." It is ironic because to be in the Olympics is Finny's fantasy.